Problem: Which of these numbers is composite? ${2,\ 41,\ 61,\ 83,\ 90}$
Explanation: A composite number is a number that has more than two factors (including 1 and itself). 2, 41, 61, and 83 each have only two factors. The factors of 90 are 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, and 90. Thus, 90 is the composite number.